In equipment run by batteries it is important to minimize the power consumption in order to prolong the battery lifetime and thus the operating time of the equipment. The power consumption of electronic equipment can be affected in many different ways. In particular, devices with CMOS circuits are characterized by the fact that power consumption is a linear function of the clock frequency. Furthermore, many devices such as radiotelephones have several local clock frequencies for different parts or circuits of the device. Also, different blocks of the circuits such as integrated circuits can comprise local clock frequencies. Continuous clocking of these circuits and blocks by a high constant clock frequency, even when the operation of a circuit or a block is low, causes unnecessary power consumption. The need for processing power varies largely in mobile phones of cellular systems. For instance, with the telephone in standby the need for processing power is small, whereas in active mode it can be 10 to 100 times greater. It is obvious that the radio telephone must provide the circuits with the required processing power, otherwise it will not operate.